refactor: type the example source inline instead of shipping a manifest

A dora-node.yml describes exactly one node and is the unit `dora hub publish`
operates on, which expects it beside pyproject.toml. This repository ships one
node, so it should carry one manifest. examples/dora-node.yml described a test
fixture that nobody installs and that has no package of its own -- a manifest
for something that is not a node.

The source's outputs are now annotated with `output_types:` in the dataflow,
which is the documented way to type an output without a manifest: "only
annotated outputs are type-checked". It runs as `path: python` + `args:`
because `dora validate` matches a manifest to a `path:` node by entrypoint, so
a path node would be tried against the root manifest, fail to match
`dora-image-view`, and warn -- fatal under strict_types. Through the
interpreter there is nothing to match.

The type gate is unchanged in strength: the viewer's contract is still
injected and compared against the source's annotations, and a deliberate
Float32 -> String swap still fails validate.

The cost is that the fixture no longer gets a managed venv of its own and runs
against the project environment. That applies to the fixture only; the shipped
node still uses path: + build:, which is what a consumer exercises.

This supersedes the approach in "fix strict-type validation by giving the
example source a manifest" and the doc commit that cited the
manifest-next-to-a-path-node rule as justification.

dataflow.yml

@@ -11,7 +11,21 @@ strict_types: true
 
 nodes:
   - id: source
-    path: examples/source.py
+    # Run through the interpreter rather than as `path: examples/source.py`.
+    #
+    # A dora-node.yml describes exactly one node and is what gets published
+    # (`dora hub publish` expects it beside pyproject.toml), and this
+    # repository ships one node: dora-image-view. The example source is a test
+    # fixture, not a node anyone installs, so it gets no manifest.
+    #
+    # That has a mechanical consequence. `dora validate` matches a manifest to
+    # a `path:` node by entrypoint, so a path node here would be tried against
+    # the root manifest, would not match its `dora-image-view` entrypoint, and
+    # would warn -- which strict_types turns into a failure. Going through the
+    # interpreter leaves nothing for it to match, and `output_types` below is
+    # the documented way to type an output without one.
+    path: python
+    args: examples/source.py
     # The source needs dora-rs, numpy and pyarrow; installing the project gives
     # it exactly the versions pinned in uv.lock.
     build: pip install .
@@ -23,6 +37,13 @@ nodes:
       - camera_info
       - bbox
       - text
+    # bbox is deliberately absent: it is untyped on the viewer's side too,
+    # because senders disagree on 32- vs 64-bit numeric fields.
+    output_types:
+      image: std/core/v1/UInt8
+      text: std/core/v1/String
+      depth: std/core/v1/Float32
+      camera_info: std/core/v1/Float32
 
   - id: view
     path: dora-image-view